1. Field of the Invention
The field of the invention relates to use of computer methods for validating the operational durability of an electronic circuit card product, composed of or containing one or more circuit card assemblies, based upon physics-of-failure analysis and fatigue damage correlation of accelerated laboratory vibration tests.
2. Discussion of Prior Art
During its design life, an electronic product can be exposed to a wide variety of vibration and shock environments. Stress screening is a process used for verification of production quality, in which systems are subjected to environmental load conditions. Vibration in stress screening processes is a very effective means of production quality verification. In order to reduce testing time and cost, laboratory tests are usually conducted on products in a time-accelerated manner. Testing programs have been historically supported by detailed analysis using finite element programs. Electronic systems are very complex structures with characteristics that make accurate predictive analysis nearly impossible.
Prior art methods of determining the adequacy of electronic designs have included the use of extensive test programs, sometimes supported by detailed finite element analysis. When failure occurs under vibration, the failure is a fatigue failure of one or more components of the system. Fatigue is a mechanical failure that occurs in materials that are cyclically loaded, usually occurring as a result of the growth of a crack in the material under repeated load reversals. A wide distribution of life capabilities can be expected when electronic systems are vibration tested to failure. Due to variations in so many contributing factors including material properties and dimensional tolerances, testing programs to fully understand product capabilities must be very extensive. Since a test only provides pass or fail information, testing must be extensive since it must be capable of finding all of the possible failure parts of the system. Since parts typically vary significantly in the parameters that effect failure rate, the defined test procedure must include variations that accommodate the expected variations in properties of critical parts. Historically, test programs have often failed to provide information critical to understanding the vibration life of the product.
Application of conventional detailed finite element analysis can aid in the knowledge gained in these testing programs. But detailed analysis is also very time consuming and expensive. There are wide variations in properties (material and dimensional) that effect life capabilities. Often, detailed analysis is thought to be accurate beyond its actual capability for the variations typical in electronic systems. Detailed analysis for vibration fatigue of electronic systems with predictive methods has very high error rates.
Testing with or without detailed finite element analysis support can be concluded based on budget considerations without an adequate understanding of the product""s real-life capabilities.
Prior art includes:
xe2x80x98Test onlyxe2x80x99 methods
xe2x80x98Detailed analysis onlyxe2x80x99 methods
Combined test/analysis methods
xe2x80x98Test onlyxe2x80x99 methods suffer from lack of detailed information on all parts and don""t define the risk for all possible failures. References 1 and 2, U.S. Pat Nos. 567,098 and 5,744,975, are examples where test only methods might be used. These test methods can be enhanced when supported by detailed analysis. U.S. Pat. Nos. 5,847,259 and 596,816 specifically include analysis of test results, but these analyses emphasize the statistical analysis of results of tests on many samples, not detailed analysis of the product being tested to understand the failure at the root cause level. U.S. Pat. No. 6,035,715 is an example of using test methods in a trial and error basis, using tests to cause repeated failures to increase capabilities by redesign to eliminate the weakest links.
Analysis only methods suffer from high error that is inherent in the parts used in electronic systems, but do provide information for all parts. However, the accuracy of the analysis for each part is unknown and creating analysis with an adequate margin to cover all error may be too conservative of a method in order to meet design requirements. Combined test and analysis methods are the best means of obtaining an understanding of the electronic system, but prior art analysis methods were extremely expensive and the analysis methods required expertise not available at all organizations in development of electronic systems.
In order to develop a reliable product using prior art methods, designs had to be developed with margins capable of extrapolating test/analysis results to cover configuration variations and inherent modeling error.
The primary objective of the computer program and method presented is to understand the effects of vibration on electronic products. Electronic products are exposed to vibration as part of normal life cycle conditions. Vibration is also used in qualification and reliability test procedures.
An additional objective is to determine a numerical definition of fatigue damage at component level based on the physics-of-failure and damage equivalence such that the vibration damage generated in any test condition can be compared to all other combinations of conditions. A method of removing computational error associated with the complex field of vibration life of electronics is used so that the associated analysis yields product understanding. Methods provide a numerical definition of damage at component part level that benefits from all previous experience, not just from designs of similar configuration or vibration excitation level. Comparison of component level damage determines if a component is at risk of failure for a defined requirement, or if it is effectively screened under environmental testing programs. The method uses the benefits of the circuit card""s natural shock and vibration modal isolation that protects most of the component parts.
The prior art method of xe2x80x98test onlyxe2x80x99 is very expensive and suffers from a lack of detailed information. The prior art methods using xe2x80x98analysis onlyxe2x80x99 require a high level of expertise, are very expensive, and lack accuracy due to high error inherent in electronic system designs. Prior art for combined test and analysis methods require a high level of expertise primarily due to the mathematical modeling in the analysis. The combined methods are very expensive and the prior art analysis methods suffer from a high level of inaccuracy.
The present invention analysis methods are greatly simplified, with lower requirements on level of expertise. The methods provide the capability of performing analysis in support of test programs with removal of error typical in analysis of electronic systems. The methods are efficient, resulting in lower costs of the combined test and analysis programs.
The one sure method of defining capabilities is testing a product to failure. This defines a product""s limits, but detailed analysis is required to provide a numerical definition to the product at point-of-failure level. The present invention provides product understanding efficiently and accurately.
The present invention relates to a computer program and method for validating the operational durability of an electronic product based upon physics-of-failure analysis and fatigue damage correlation of accelerated laboratory vibration tests.
The application program and method for determining the fatigue life fraction used during vibration testing of an electronic product, includes the application program steps of
(i) describing the configuration of the electronic product including layout with card geometry, component location with pin attachment points and by entering structural properties and support conditions;
(ii) describing the vibration exposure including sine dwells, sine sweeps and/or random vibration profiles representing environmental exposure for either accelerated life profiles or environmental screening profiles;
(iii) a program means for calculating natural response modes of the circuit card including natural frequency and mode shape;
(iv) a program means for calculating the stress functions for each component and response mode, including stresses from inertial loading and forced modal displacement;
(v) a program means for calculating the normalized fatigue damage of each component by integrating the ratio of xe2x80x98the number of experienced cycles of exposure at a stress peakxe2x80x99 to xe2x80x98the number of cycles to failure at that stress peakxe2x80x99 over the full stress response range for all defined vibration requirements;
(vi) removing computer calculation error by comparing the normalized fatigue damage for all components to the expectations of capability based on component type and component quality;
(vii) for the case of accelerated life testing using the expectations ratio to determine the adequacy of the design, or in the case of product reliability testing using the expectations ratio to determine the effectiveness of the environmental screen;
(viii) a program means for entering actual test response levels;
(ix) a program means for defining the expectations of components by detailed xe2x80x9cnormalizedxe2x80x9d analysis of actual tests to failure.